A conventional disc pad for a disc brake is molded by bonding a pad, that is a friction material, to a backing plate. The disc pad is pressed against a rotating disc to control the rotation of the disc through a friction force, thereby stopping an automobile. In this case, the disc pad is subjected to strong forces causing shearing stress and frictional heat. The disc pad is directly exposed to the environment so that the pad is directly affected by wind and rain and such snow melting agents as NaCl and CaCl.sub.2. Accordingly, the disc pad is exposed to very severe operating conditions caused by mechanical forces and corrosion.
In spite of these rough operating conditions, normally such brake pads must not accidentally fail for safety's sake. Accordingly, the surfaces of the pad and of the backing plate to be bonded to each other are required to have such a bonding strength that a failure of the bond between the surfaces is avoided.
Therefore, conventionally, the bonding of the pad to the backing plate is mechanically strengthened by roughening the backing plate surface by shot blasting prior to bonding. While this method is effective when the pad is new, the periphery or the spigot hole portion of the backing plate starts rusting during prolonged use or in case of travelling on a road with melting snow including salt. The rust gradually spreads in the bonded surface of the backing plate, resulting in the detachment of the pad from the backing plate or in a reduction of the bonding strength, causing the brake to malfunction which might lead to an accident, if the control over the vehicle is thereby lost.
Therefore, methods of preventing rusting between the backing plate and the pad have been proposed. These methods are disclosed in, for example, Japanese Patent Publication No. 57-45939, Japanese Patent Laying-Open No. 51-24533 and Japanese Patent Laying-Open No.: 59-144836.
Japanese Patent Publication No. 57-45939 discloses a technique of forming a tri-iron tetroxide layer of 0.1-5.0 .mu.m in film thickness on a surface of a backing plate. However, it is not possible by this method to attain any remarkable anticorrosiveness that is required for achieving the necessary reliability of the disc pad.
Japanese Patent Laying-Open No. 51-24533 discloses a technique of improving the anticorrosiveness of the backing plate by subjecting the same to a soft nitriding gas treatment. However, the known method involves a complicated process and it is not possible to achieve a useful cost reduction nor a remarkable anticorrosiveness.
Japanese Patent Laying-Open No. 59-144836 discloses an improvement in the anti-corrosiveness by applying to the surface of the backing plate, a chromic acid treatment with a liquid containing 5 weight % of Zn particles having a particle diameter of 5 .mu.m. However, while thic chromic acid treatment improves the anticorrosiveness, it is difficult to perform the treatment with the chromic acid for safety, hygiene and pollution reasons. Besides, the treatment might cause high manufacturing costs.
As described above, various methods are proposed for preventing rusting between the backing plate and the pad.
However, it is difficult by any of the known methods to achieve such anticorrosiveness that a highly reliable disc pad is obtained and even if such a reliable pad is obtained, there are problems in safety, hygiene, pollution, and manufacturing costs.
A conventional disc pad is integrally molded by bonding a backing plate and a lining to each other by an adhesive, which is extended due to heat and pressure applied during the molding of the friction material, whereby the adhesive is absorbed by the friction material. Thus, it is substantially impossible to specify a thickness of the adhesive layer. Even if the thickness of the adhesive is increased, it is all the same difficult to specify the thickness because of flow of the adhesive before the curing thereof. Accordingly, it is difficult to expect an adhesive to be anticorrosive in a disc pad which is one example of a conventional friction member. In addition, even a conventional disc pad or friction member treated so as to prevent rusting, has an insufficient anticorrosiveness between the backing plate and the pad, whereby it is difficult to maintain the shearing strength of the friction member for a long time.